1. Field of the Invention
The present invention relates to a fluid coupling, and more particularly to a fluid coupling which is used for controlling rotational speeds of a boiler feed pump, a descaling pump or the like in a thermal power plant, an iron production plant or the like, and is operable at high speeds.
2. Description of the Prior Art
Recently, there have been demands for variable speed regulators for operating high-speed, large-capacity pumps and blowers with high efficiency as energy saving apparatus for use with rotary machinery. To meet such demands, there has been developed a fluid coupling assembly which comprises speed-increasing gear trains and a variable-speed fluid coupling which are integrally housed in a casing, the variable-speed fluid coupling having an impeller runner assembly mounted on a high-speed shaft of the speed-increasing gear train.
FIG. 2 of the accompanying drawings shows a conventional fluid coupling. As shown in FIG. 2, the conventional fluid coupling has an input shaft 21 with a large gear 22 mounted thereon, and a drive shaft 23 disposed parallel to the input shaft 21 and supporting a small gear 24 held in mesh with the large gear 22. The large gear 22 and the small gear 24 which are held in mesh with each other jointly serve as a speed-increasing gear train for increasing the rotational speed of the drive shaft 23 as compared with the rotational speed of the input shaft 21. The conventional fluid coupling also has a driven shaft 25 disposed adjacent to and coaxially with the drive shaft 23.
An impeller 26 is coupled to an inner end of the drive shaft 23, and a runner 27 is coupled to an inner end of the driven shaft 25. The impeller 26 is fixed to an impeller casing 28 which houses the runner 27 therein. The impeller 26, the runner 27 and the impeller casing 28 jointly define a working fluid chamber with a scoop tube 30 disposed therein for continuously varying the rotational speed of a load such as a pump coupled to the fluid coupling from a minimum speed level to a maximum speed level.
A large gear 31 is mounted on the driven shaft 25 and held in mesh with a small gear 33 mounted on an output shaft 32 which extends parallel to the driven shaft 25. The large gear 31 and the small gear 33 also jointly serve as a speed-increasing gear train for increasing the rotational speed of the output shaft 32 as compared with the rotational speed of the driven shaft 25.
Auxiliary machines such as a main oil pump 36 and an auxiliary oil pump (not shown) are connected to the input shaft 21 through gears 35. The main oil pump 36 axially extends in a direction normal to the sheet of FIG. 1.
If the rotational speed of the load such as the pump is high so as to be in the range of 7,000 to 10,000 rpm, the conventional fluid coupling meets such a high speed requirement by increasing the rotational speed with the two speed-increasing gear trains, positioned respectively on input and output sides of the impeller 26 and the runner 27, such that the peripheral speeds of effective-diameter portions of the impeller 26 and the runner 27 will not exceed a certain reference value which normally ranges from about 150 m/s to 165 m/s.
Since the peripheral speeds of effective-diameter portions of the impeller 26 and the runner 27 will not exceed a certain reference value which normally ranges from about 150 to 165 m/s, the impeller 26, the runner 27 and the impeller casing 28 are protected from fracture or fatigue-induced fracture due to various stresses including centrifugal stresses developed in the impeller 26, the runner 27 and the impeller casing 28 when power is transmitted thereto, stresses and thermal stresses developed by a centrifugal hydraulic pressure in the working fluid chamber, and stresses caused when torque is transmitted. Since outer circumferential open ends of the impeller 26 and the impeller casing 28 are of an overhanging and cantilevered structure, the peripheral speeds of the impeller 26 and the impeller casing 28 are suppressed because their outer circumferential open ends are most subject to fatigue-induced fracture. For the above reasons, the two speed-increasing gear trains are disposed respectively on the input and output sides of the impeller 26 and the runner 27 to meet rotational speed requirements of the load such as a pump.
The two speed-increasing gear trains, however, have suffered various disadvantages in that the fluid coupling is large in overall size and weight, takes up a large installation space, and is highly costly to manufacture.